How long does an ammunition transit case prototype take?

Answer: About fourteen days from trial shot to signed first article. The time goes into letting the part stabilise before measurement and into the second trial round.

Why measure five thickness points instead of the thickest one?

Answer: The spread drives internal stress. Readings within 0.4 mm mean the part cools evenly, which stops a crack at the thick-to-thin transition.

Why charge to 3 kPa rather than something higher?

Answer: 3 kPa already exposes a leak anywhere on the gasket. Push higher and the gasket is forced into the groove floor, flattering an assembly that never sees that load.

Can the drop test be dropped from the sheet?

Answer: Better not. Stacking proves static strength; dropping proves the latch stays shut under impact, a different part doing a different job.

Fourteen Days: Building an Ammunition Transit Case Prototype

Quick answer: A build log of one ammunition transit case with a 620 × 420 × 230 mm cavity, from trial shot to signed first article: the thickness points, decay limit and drop criteria that decided it.

Where the Ammunition Transit Case Started
Five Wall Thickness Points, Not One
Sealing Starts With a Pressure-Decay Curve
Stacking and Drop Test Answer Different Questions
Six Lines Worth Signing on the First-Article Sheet
FAQ

1. Where the Ammunition Transit Case Started

The customer brought no sample, only a cavity drawing: 620 × 420 × 230 mm clear, twelve cartons, 18 kg loaded, three tiers on his rack.

We tabulated the inputs before arguing about structure; nothing was estimated.

Input	Value	Source
Internal cavity	620 × 420 × 230 mm	Packing list
Loaded weight	18 kg	Twelve cartons weighed
Stacked tiers	3	Rack upright spacing
Trips per year	40	Warehouse logbook

The line people skip is 40 trips a year: 320 closures over eight years, each pushing the latch through one elastic cycle. Prototypes obsess over wall thickness; the latch boss fails first.

2. Five Wall Thickness Points, Not One

The drawing gives one thickness. The mould gives five.

Ammunition Transit Case: black case on a wooden workbench, key in the lock between two latches, folding handle at the edge, timber shelves behind

The first article came off a 1250-tonne press, cooled 38 seconds and rested 24 hours before gauging. Five readings per face; a 0.4 mm spread points to a difference between the two tool halves.

Point	Measured	Drawing	Carries
Centre of base	6.0 mm	6.0 mm	Vertical load when stacked
Mid side wall	5.5 mm	5.5 mm	Side squeeze, forklift knocks
Latch boss	9.0 mm	8.5 mm	Repeated closure
Rib root on lid	7.2 mm	7.0 mm	Spring-back after a drop

Only the latch boss missed: 8.5 specified, 9.0 measured. That looks like strength but is the front edge of a sink mark, a hollow daylight never shows but a 300 mm steel rule will.

The fix was a 1.5 mm relief groove behind the boss, splitting one thick zone into two 4.5 mm ones that cool alike; the second shot read 8.6 mm.

Thickness on an ammunition transit case is not about more being better. Every zone has to cool at one speed, because a step in thickness becomes a crack two years later.

3. Sealing Starts With a Pressure-Decay Curve

A seal is not about pressing harder, but pressing just enough.

Charge the closed case through the valve to 3 kPa, shut it, wait 30 seconds. Our limit is 0.3 kPa; the first article read 0.15, meaning the gasket held a continuous ring with no ongoing leak. For an ammunition transit case the groove is the root: 6 mm wide, 4 mm deep, 7 mm gasket, about 30 percent compression. Under 20 percent, rain climbs the groove floor; over 45 percent, the gasket takes a set and never seals again. After a cold test at -30 °C pushed closure force from 180 N to 260 N, needing two hands, we deepened the groove from 4 mm to 4.5 mm, and force fell to about 200 N. That change never reached the customer drawing.

4. Stacking and Drop Test Answer Different Questions

Stacking and dropping are separate tests, and neither substitutes for the other.

Ammunition Transit Case: black case resting on a diamond-plate truck bed, rear doors open, keys lying beside it, wooden pallet and a brown strap behind

For stacking, 300 kg rests on the lid for 24 hours. Residual deflection at the centre must stay under 3 mm, and the first article read 1.8 mm. That figure is derived: three tiers means the bottom case carries two above, 18 kg doubled twice for margin.

The drop test on this ammunition transit case runs on concrete from 1.2 m, eight orientations, at the full 18 kg. No shell crack, no latch popping, no white stress line through the cavity. On the fourth corner the latch popped, because the hook had a 25-degree lead-in that the impact ramp lifted straight out. At 35 degrees with a throat 0.8 mm deeper, eight repeats passed.

5. Six Lines Worth Signing on the First-Article Sheet

Six lines on the sign-off sheet carry real weight.

Cavity clearance in three axes, 620 × 420 × 230 mm, tolerance ±2 mm
Five thickness points, spread within 0.4 mm on any face
Pressure decay from 3 kPa, no more than 0.3 kPa in 30 seconds
Latch retention above 300 N once engaged
300 kg for 24 hours, lid deflection under 3 mm
Eight drops from 1.2 m, no crack and no popped latch

The rest is process record: mould curves, trial counts, why each change was made. Round three is the one to remember. A fixed handle instead of a folding one, because gloves could not grip the fold, pushed weight from 6.83 kg to 7.1 kg, over the 7 kg ceiling, until the handle rib went from 4 mm to 3.2 mm and weight returned to 6.95 kg.

The serial number is moulded into the right rear corner of the lid at 8 mm character height, matching the ledger digit count, because a label is gone by the third handling.

Conclusion

Only three things changed shape in fourteen days: the relief groove behind the latch boss, half a millimetre of groove depth, and a latch hook opened to 35 degrees. Together they decide whether this ammunition transit case still closes tight after 320 openings and 40 trips a year.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.